A popular method of displaying information is by the use of overhead transparencies. Clear plastic sheets are imprinted with information from a variety of sources, such as photocopies, desktop plotters, and hand-drawn images using ink pens. These images are then placed on an overhead projector for display. The overhead projector passes high-intensity light through the transparent images, through a set of optics and onto a screen.
An overhead projector commonly includes a light box having a horizontal cover glass, and a lens and mirror arrangement positioned above the cover glass for projecting onto a remote viewing screen the image of a transparency positioned on the cover glass and illuminated by a light source located within the box. Usually, the changing from one transparency to the next is performed manually, with each transparency being manually placed in turn onto the light box cover glass for viewing, and then manually removed after viewing. Such procedure is not only tedious and tiresome, but makes it difficult to maintain the transparencies in proper order, particularly if some of the transparencies will be shown multiple times during the same presentation. Moreover, in order to have correct image orientation during viewing, each transparency must be correspondingly correctly oriented on the viewing plate. This makes handling even more cumbersome, especially if the room is darkened to enhance image contrast.
Accordingly, attempts have been made to construct power-operated changers for automatically feeding a stack of transparencies, one at a time, successively onto the cover glass of an overhead projector. Examples of such changers are disclosed in U.S. Pat. Nos. 3,110,217; 3,352,202; 3,594,082; and 4,756,616 (the disclosures of which are incorporated herein by reference thereto). In such prior art changer mechanisms, the transparencies to be viewed are stacked in a supply bin located on one side of a projector. They are automatically successively transported, one transparency at a time, to a viewing station in registration with the projector cover glass and then, following projection, are delivered to a receiving bin.
The conventional changers are complex and bulky. Supply and receiving bins are normally separate elements, awkwardly arranged to stick out in generally horizontal projection, often from opposite sides of the projector. Transparencies are stacked in horizontal orientation, parallel with the plane of the cover glass. And, even when bidirectional provision is made so that feeding of transparencies can occur both from the supply bin to the receiving bin and vice versa, the next transparency is always selected from or returned to the same side (top or bottom) of the stack in any one bin.